Device for preventing electrically induced fires in gas tubing

ABSTRACT

The present invention provides a device for preventing electrically induced gas fires involving gas tubing such as Corrugated Stainless Steel Tubing (CSST) and gas appliance connectors (GAC). Connectors for coupling the gas tubing to gas lines and appliances are affixed at each end of the tubing. These connectors are made of a conductive material. Conductive wire (i.e. mesh surrounding the gas tubing) provides a direct electrical contact between the end connectors. If an electrical charge goes to ground via the gas tubing, such as from a lightning strike or an appliance short, the current is carried between the end connectors by the conductive wire instead of the gas tubing itself, thereby preventing damage to the gas tubing from the current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/183,527 filed Jul. 18, 2005 (now U.S. Pat. No. 7,562,448). The entiredisclosure of the prior application is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates generally to the prevention of firescaused by lightning and more specifically to fires involving gas leaksin Corrugated Stainless Steel Tubing and similar gas lines (sometimesreferred to as appliance connectors).

BACKGROUND OF THE INVENTION

Corrugated Stainless Steel Tubing (CSST) is a relatively new buildingproduct used to plumb structures for fuel gas in lieu of conventionalblack pipe. The advantages that are offered for CSST include a lack ofconnection and a lack of threading. In essence, it is a material thatresults in substantial labor savings relative to using black pipe.

CSST differs from black pipe in a number of ways. In a CSST system, gasenters a house at a pressure of about 2 psi and is dropped to ˜7″ WC bya regulator in the attic (assuming a natural gas system). The gas thenenters a manifold and is distributed to each separate appliance via“home runs.” Unlike black pipe, a CSST system requires a separate runfor each appliance. For example, a large furnace and two water heatersin a utility closet will require three separate CSST runs. With blackpipe, the plumber may use only one run of 1″ pipe and then tee off inthe utility room. Therefore, the requirement of one home run perappliance significantly increases the number of feet of piping in abuilding.

CSST is sold in spools of hundreds of feet and is cut to length in thefield for each run. In this regard, CSST has no splices or joints behindwalls that might fail. CSST also offers an advantage over black pipe interms of structural shift. With black pipe systems, the accommodationsfor vibrations and/or structural shifts are handled by applianceconnectors, a form of flexible piping.

However, the thin flexible walls of CSST have a propensity to fail whenexposed to electrical insult, particularly lightning. CSST is very thin,with walls typically about 10 mils in thickness. The desire for easyrouting of the tubing necessitates this lack of mass. However, it alsoresults in a material through which electricity can easily puncture.

Once the tubing has been perforated, it is possible for the escaping gasto be ignited by the metallic by-products of the arcing process, byauto-ignition, or by adjacent open flames.

The theoretical energy level required to melt a specimen can be comparedby using both heat capacity and melting temperature. The heat capacityis the amount of heat needed to raise the temperature of either sampleone degree Celsius. Changing the temperature from an initial to themelting temperature requires the heat capacity to equal:q=C·m·ΔT _(m) +m·H _(f)where:

C is the specific heat

H_(f) is the heat of fusion

m is the mass of the specimen

ΔT_(m) is the change from the initial temperature to the meltingtemperature

Based on this equation, one can derive theoretical values for heatcapacity for an equivalent 100 mil diameter hole in different materials.The amount of energy required to create a 100 mil diameter hole is muchhigher for black pipe than other pipe materials including aluminum,copper and CSST. The amount of energy required to melt a conventionalhalf inch black pipe is about 15 times the energy required to similarlymelt CSST, about 10 times the energy required for aluminum and five timethat for copper. Thus, the thickness of the pipe plays a critical role.

Field data indicates that lightning damage to black pipe is sometimes sosmall that it is often only visible with microscopic analysis andlimited to a small pit that does not leak. Lightning strikes involvingCSST create leaks that vary from pinhead size to almost quarter inchholes.

Lightning strikes vary in current from 1,000 (low end) to 10,000(typical) to 200,000 (maximum) amperes peak. Mechanical damage caused byheating is a function of the current squared multiplied by time. Thus,the current is the dominant factor creating the melting of gas tubing.

One of the underlying issues with CSST is that it is part of theelectrical grounding system. For reasons of electric shock prevention(and also elimination of sparks associated with static electricity), itis desirable to have all exposed metal within a structure bonded so thatthere are no differences of potential. However, there are limitations toapplying DC circuit theory (or even 60 Hz steady state phasor theory) inthis situation because lightning is known to have fast wavefronts. Whilethe reaction of large wires and irregular surfaces is predictable at 60Hz, the fast wave fronts associated with lightning may cause substantialproblems with CSST, given its corrugated surface. Moreover, new houseconstruction has shown very tight bends and routing of CSST immediatelyadjacent to large ground surfaces, creating the potential for arcscreated by lightning strikes. Testing of CSST under actual installedconditions using transient waveforms may well show further limitationsthat conventional bonding and grounding cannot accommodate.

The typical gas line or gas system, whether black pipe or CSST, isusually not a good ground. The metal components that make up a gas trainare made from materials that are chosen for their ability to safelycarry natural gas (or propane) and the accompanying odorant. Thesemetallic components are not known for their ability to carry electriccurrent. To further compound matters, it is not uncommon to find pipejoints treated with Teflon tape or plumber's putty, neither of which isconsidered an electrical conductor. The Fuel Gas Code (NFPA 54) callsfor above ground gas piping systems to be electrically continuous andbonded to the grounding system. The code provision also prohibits theuse of gas piping as the grounding conductor or electrode.

Appliance connectors, which are prefabricated corrugated gas pipes, arealso known to fail from electric current, whether this current is fromlightning or from fault currents seeking a ground return path. Theseconnectors usually fail by melting at their ends (flares) during timesof electrical overstress. These appliance connectors are betterdescribed ANSI Z21.24, Connectors for Indoor Gas Appliances, thecontents of which are hereby incorporated by reference. A gas appliancethat is not properly grounded is more susceptible to gas line arcingthan a properly grounded appliance. The exact amount of fault current,however, will depend upon the impedances of the several ground paths andthe total fault current that is available. For example, air handlers forold gas furnaces seem to be the most prone. Typically, an inspectionwill reveal that the power for the blower motor uses a two-conductor(i.e. non-grounded) power cord.

A primary indicator that is found in these types of fires is the focalmelting of the gas line at the brass nut/connector. It is well known andappreciated that the flame that is fueled from a gas orifice does notnormally make physical contact with the orifice itself. Rather, there issome distance between the flame and orifice depending on the gaspressure, the size of the orifice, available oxygen, and the mixing orturbulence. In short, the leaking gas is too rich to burn at the pointof escape. In addition, gas that is under pressure will cause a verysmall amount of cooling to occur when the gas escapes from such a leakor orifice due to adiabatic cooling. Both of these factors indicate thata gas line would be least likely to melt at a connection if the meltingwere indeed caused by the heat from a flame, as opposed to electricalinsult.

For the several above described reasons, it would be desirable to have aCSST system or a gas appliance connector (GAC) that is capable of carrycurrent in the case of electrical arcing or carrying fault currentswithout suffering perforation or melting at the connector end, therebypreventing the leaking and ignition of gas fuel. It is further desirableto have a CSST or GAC function in such a way that minimizes the chancesof having perforations created on its side walls due to electric injury(i.e. from lighting and similar phenomenon).

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for preventingelectrically induced gas fires involving Corrugated Stainless SteelTubing (CSST) and appliance connectors. Connectors for coupling the CSST(or gas appliance connectors) to gas lines and appliance are affixed ateach end of the tubing. These connectors are made of a conductivematerial such as brass. Conductive ground wire, such as #8 AWG copper,provides a direct electrical contact between the end connectors. Theground wire can be either a single wire or a wire mesh; in mesh form,the mesh serves as an outer shield for the gas tubing. If an electricalcharge goes to ground via the CSST or the appliance connector, such asfrom a lightning strike or an appliance short, the majority of thecurrent is carried by the conductive wire rather than the CSST orappliance connector itself, thus preventing damage to the CSST from thecurrent. When using a mesh type shield, if the CSST or applianceconnector receives an electrical charge from arcing to the side walls,the mesh serves as a current shunt and thus both shunts the current andcauses the charge on the actual CSST (or appliance connector) wall to bedissipated over a larger area.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, however, as well asa preferred mode of use, further objects and advantages thereof, willbest be understood by reference to the following detailed description ofan illustrative embodiment when read in conjunction with theaccompanying drawings, wherein:

FIG. 1 shows a partial cross section a house illustrating the mechanicalconnection between the gas line, furnace and air conditioning system;

FIG. 2 illustrates another scenario for a CSST related gas fire in whichthe fires is induced by an electrical short from an appliance;

FIG. 3 shows yet another situation in which electrical grounding candamage CSST lines;

FIG. 4 depicts an example of a CSST perforation caused by electricalarcing;

FIG. 5 shows a cross section view illustrating the physical interfacebetween a CSST and gas pipe;

FIG. 6A shows a CSST with a single grounding wire in accordance with thepresent invention; and

FIG. 6B shows an alternate embodiment of the present invention thatemploys multiple strands of grounding wire.

DETAILED DESCRIPTION OF THE DRAWINGS

FIGS. 1-4 illustrate common scenarios for electrically induced gas firesinvolving Corrugated Stainless Steel Tubing (CSST).

FIG. 1 shows a partial cross section a house illustrating the mechanicalconnection between the gas line, furnace and air conditioning system. Inthis example, the furnace 101 is located in the attic of the house 100.The air conditioning unit 102 is located at ground level. Gas from thegas main 110 enters the house 100 through a feeder line 111. A CSST line120 connects the feeder 111 to the furnace 101.

The metal chimney 102 of the furnace 101 extends through the roof. Ifthis chimney 103 is struck by lightning 130, the charge will often go toground through the CSST line 120 as indicated by arrow 140.

FIG. 2 illustrates another scenario for a CSST or gas applianceconnector related gas fire in which the fire is induced by an electricalshort from an appliance. FIG. 2 shows an arrangement similar to that inFIG. 1 involving a CSST line 201, a furnace 202 and an A/C unit 203. Ifthe A/C motor 203 becomes stuck the windings in it burn out and short toground though their physical connection to the furnace 202 and CSST line201 as indicated by arrows 210, 211.

FIG. 3 shows yet another situation in which electrical grounding candamage CSST lines. In this example, a tree 320 has fallen across twopower lines 301, 302 connected to a house 310. The tree 320 causes thehigh volt line 301 and the ground line 302 to touch together. In thissituation the ground line 302 becomes energized and spills currentthrough the entire house 310, which can result in the electrical currentgrounding through CSST lines as illustrated in FIGS. 1 and 2.

FIG. 4 depicts an example of a CSST perforation caused by electricalarcing. In this case, the CSST 430 runs parallel to a metal chimney 401but is not in direct physical contact with the chimney. If the chimney401 is struck by lightning 410, the potential difference created by thelightning strike might be large enough to produce an electrical arc 420between the chimney and the CSST 430. Such electrical arcing is mostlikely to produce perforation along the length of the CSST.

FIG. 5 shows a cross section view illustrating the physical interfacebetween a CSST and gas pipe. Flexible appliance connectors, asrecognized by the Fuel Gas Code and other codes, make use of flaredconnections at their ends 501, along with the usual nut 502 (oftenbrass) to make the connection secure. One means of failure of thesetypes of connections is brought about when current from electricdischarges is sent down the appliance connector in an attempt to reachground potential. As discussed above, examples of such dischargesinclude lightning, floating neutral conditions, grounding out of airconditioning compressors, and energizing of ground circuitry byinadvertent shorting within appliances.

Another mode of failure occurs when during a “ground out event” thecharge passes from the side of a connector to an adjacent metal surface.For example, if current is flowing through the gas line and the gas lineis touching (on its side) the steel chassis, electricity may arc duringthe shorting process from the brass (nut) or stainless steel of the gasline to the steel chassis, causing a gas leak and resultant fire.

While the flared connections 501 are sufficient in terms of theirability to carry gas from a mechanical connection, the flared connectionis subject to failure when required to carry electric current. Theelectric current often causes the flared connection to melt and arc,resulting in a gas leak and igniting the gas.

FIG. 6A shows a CSST with a single grounding wire in accordance with thepresent invention. The present invention reduces the risk of the typesof failures described above by placing a copper ground wire 601 of ˜#8American Wire Gauge (AWG) or larger electrically in parallel with thelength of CSST or the appliance connectors 610,620. This ground wire 601attaches to the end connectors 610, 620 at the set screws 611, 621, oneon each end.

When the connector is fabricated at the factory, the brass nut on oneend has a set screw and collar. The #8 wire is secured in this hole atthe factory. On the opposite end of the connector, there is a similarcollar and set screw. However, the loose end of the copper wire is notattached to the second nut until the nut has been tightened and allconnections are leak tested. Thereafter, the parallel wire conductor canbe threaded into the appropriate collar and held in position by the setscrew.

FIG. 6B shows an alternate embodiment of the present invention thatemploys multiple strands of grounding wire. In this embodiment, thebrass nuts 610, 620 have collars 630, 640 that protrude from the top.The CSST flex line 600 is shrouded by a wire mesh 650, which is attachedto the collars 630, 640 on each end.

The wire mesh shielding 650 terminates on each collar 630, 640 in amanner that allows the shielding to make electrical and mechanicalcontact with the nuts 610, 620 but allows the nuts to be rotated freelyabout the end of the flare and the coax.

Each of the embodiments of the invention depicted in FIGS. 6A and 6Bkeeps electrical current from damaging the flared ends of the CSST byproviding an electrical shunt in the form of copper ground wire betweenthe brass connectors on the ends. Since copper is a superior conductorto CSST, it can safely carry currents that the CSST was never designedto handle.

The braid/wire mesh technique depicted in FIG. 6B also has the benefitof mechanically protecting the connector from mechanical damage, damagefrom loose, energized conductors, damage from lightning, and damagecaused by corrosives (e.g., some housekeepers have been known to useammonia or the like on stove gas hoses).

In addition to application in new housing construction, the currentshunting system of the present invention can easily be retrofitted toexisting houses. In houses with black pipes, it is common to useappliance connectors to transition between the black pipe and theappliance; a gas appliance connector (GAC) is similar to CSST in that ithas thin walls, corrugations, and is easily bent or manipulated. Theseappliance connectors fail similarly to CSST when electrically injured.It is a simple matter to construct a GAC with the same current shuntingas the present invention proposes for CSST. Thereafter, the present gasappliance connector can be replaced with the invention herein described.

The description of the present invention has been presented for purposesof illustration and description, and is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the art. Theembodiment was chosen and described in order to best explain theprinciples of the invention, the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated. It will be understood by one of ordinaryskill in the art that numerous variations will be possible to thedisclosed embodiments without going outside the scope of the inventionas disclosed in the claims.

1. An electrical shunt for gas tubing comprising: (a) connecting meansat each end of the gas tubing for coupling the tubing to gas lines andappliances, wherein the connecting means are made of a conductivematerial that has a higher conductivity than the gas tubing; and (b)conducting means for providing direct electrical contact between saidconnecting means at either end of the gas tubing, wherein the conductingmeans has a higher conductivity than the gas tubing and carrieselectrical current between the connecting means in the event ofelectrical arcing and ground faults, thereby keeping the current off thegas tubing and preventing damage to the tubing.
 2. The electrical shuntaccording to claim 1, wherein the gas tubing comprises CorrugatedStainless Steel Tubing (CSST),
 3. The electrical shunt according toclaim 1, wherein said connecting means are made of brass.
 4. Theelectrical shunt according to claim 1, wherein said conducting means ismade of copper.
 5. The electrical shunt according to claim 4, whereinthe copper is approximately number 8 American Wire Gauge (AWG) orlarger.
 6. The electrical shunt according to claim 1, wherein saidconducting means comprises a mesh surrounding the gas tubing.
 7. Theelectrical shunt according to claim 1, wherein the gas tubing comprisesGas Appliance Connector (GAC).